Shortcut Trick
Find the Least Common Multiple (LCM) of the given intervals: 5, 10, 15, 20, 25, and 30 seconds.
LCM (5, 10, 15, 20, 25, 30) = 300 seconds = 5 minutes.
Number of times the bells ring together = (Total Time ÷ LCM) + 1.
Required times = (20 ÷ 5) + 1 = 4 + 1 = 5 times.
∴ The correct answer is 5 times.
Alternate Method
Given:
Intervals of six bells = 5, 10, 15, 20, 25, and 30 seconds.
Total time duration = 20 minutes.
Formula Used:
Common interval = LCM of individual intervals.
Number of rings = (Total duration ÷ Common interval) + 1 (to include the first ring).
Calculations:
⇒ Prime factorization of intervals:
5 = 5
10 = 2 × 5
15 = 3 × 5
20 = 22 × 5
25 = 52
30 = 2 × 3 × 5
⇒ LCM = 22 × 3 × 52 = 4 × 3 × 25 = 300 seconds.
⇒ Common interval in minutes = 300 ÷ 60 = 5 minutes.
⇒ Total rings = (Total Duration ÷ Common Interval) + 1
⇒ Total rings = (20 ÷ 5) + 1 = 4 + 1 = 5 times.
∴ The correct answer is 5 times.
Additional Information
Least Common Multiple (LCM)
The smallest number that is a multiple of two or more numbers. In coincidence problems (bells, traffic lights), the LCM determines the next time all events occur simultaneously.
Commencement Rule
When the problem states "including the first ring at commencement" or "ringing together at the start", always add 1 to the division result: (Total Time ÷ Interval) + 1.
Unit Consistency
Before dividing, ensure the total time and the common interval are in the same unit (e.g., both in seconds or both in minutes).